i'm one of those people who have it, use it with success, and i've done king size quilts on it.
learning to quilt on a frame is completely different then sitting at your machine and doing FMQ.
just as you had to learn sitting FMQ you have to put in the time to learn FMQ on a frame, either flynn or any other brand.
i use this frame with a 5 inch throat and while i have limitations its not with the frame - its with the 5 inch throat machine.
if you baste your quilt and pull out the middle rod you have the ability to quilt 15 inches - so clearly a 12 inch block wouldn't be a problem if you have a machine with a large enough throat.
95% of the time i baste my quilt first, pull out the middle rod, and then i do the detailed quilting. this allows me to advance the quilt forward/back with the needle down to quilt a 12 inch design without having to cut the threads.
if you want to try frame quilting (its not for everyone) its the least expensive way to try it. if you find that you love frame quilting you will certainly want to upgrade to a larger frame and larger throat machine but that will be thousands of dollars.
for me, it was an easy decision, i didn't know if i would enjoy frame quilting so for the $150 investment it was well worth it and i've quilted about 20 quilts on the frame now.
i'm upgrade one day but as my budget permits - i don't have thousands to invest in one lump sum.